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Souma Dutta@ 10:07am 03-14-2013

College/Institute and Class

Asutosh college,2nd year
How can i identify if the mosfet is bad or not?
Replied on: 7:12pm 03-21-2013

The MOSFET is usually destroyed by static charges which accumulate on the metallic gate and lead to high voltages which exceed the Gate to Source breakdown voltage, leading to shorting of the insulating SiO2 layer and conduction of current through it. As a result a milliammeter would record a gate current > 0 unlike the normal case, where due to the very high input impedance of the device, the gate current is essentially zero.

Two back-to-back zener diodes are often used in series to clamp the voltage between the gate and source to a level less than the breakdown value.

You may refer Boylstead Nashelsky's book.

MD KURBAN@ 10:38am 03-13-2013

College/Institute and Class

Surendranath Evening College
The barrier potential across a p-n junction diode cannot be measured by placing a voltmeter across the diode terminals. Explain Why?
Replied on: 7:00pm 03-21-2013

The barrier potential across the p-n junction is a contact potential between two semiconductor regions which, when the junction is unbiased (i.e. no voltage source is connected between the p and n regions), is exactly nullified by the sum of two other contact potentials which develop across the two (ohmic) metal-semiconductor contacts (between the p region and the metal wire connecting one of the voltmeter terminals and the n region and the metal wire connecting the other voltmeter terminal).
Refer Millman Halkius' or Streetman's books for better understanding.

EJAZ AHMED@ 8:44pm 03-04-2013

College/Institute and Class

How does the free electrone concentration increase over the intrinsic value in an n-type semiconductor? Will the hole concentration remain constant at the intrinsic value? If not, why?
Replied on: 1:20am 03-12-2013

An n-type semiconductor is doped with pentavalent impurity atoms,each of which contributes a free electron to the host semiconductor. Since the doping concentration can be made very high, many such extrinsic free electrons can be made available.
Thus the number of free electrons per unit volume (n) can become much larger than the intrinsic concentration of free electrons (ni).
Now quite a few of the free electrons recombine with the intrinsic holes (there are no extrinsic holes in an n-type semiconductor).
As a result, at equilibrium, there is a net decrease in the hole concentration and an equal increase in the free electron concentration compared to their intrinsic values. This ensures that mass action law is satisfied..i.e. np=constant where the constant equals ni^2=pi^2 for the same semiconductor prior to doping.

EJAZ AHMED @ 9:35pm 02-27-2013

College/Institute and Class

Surendranath Evening College
Sir, How a semiconductor diode can be used as a rectifier?
Replied on: 1:06am 03-12-2013

This is a very basic question which must have been discussed by your teacher teaching the topic. You should also look up your text book (any book on basic electronics, for that matter!).
In any case, to understand the answer to yopur question, picturize the V-I characteristic of a forward and reverse biased p-n diode in your mind. What do you see from the graph? Under forward bias, the current increases exponentially (well almost!). Under reverse bias, negligible current flows.
When the diode is connected to an ac source through a resistor in series, for (almost) half the a.c cycle the diode is forward biased and hence conducts current. For the remaining half-cycle, the diode is reverse biased and hence is cut-off, implying no current flows through it. Since the resistor is connected in series with the diode, the current in the resistor has the same relationship with the input a.c signal. If the voltage across the resistor (V=IR) is now plotted with respect to time, you would observe the halve-wave rectifier output waveform one comes across in the text-books.
Full-wave rectifiers use two or more diodes to achieve current flow in both the a.c cycles, but in the same direction.

EJAZ AHMED@ 3:35pm 02-27-2013

College/Institute and Class

Surendranath Evening College
Sir I request you, if possible please bring the ELTA question papers of 2011 and 2012 in our next class. Because the ten years question papers are not available in the market.
Replied on: 1:22am 03-12-2013

As I told you before, you need to take the help of your seniors in matters such as these.

EJAZ AHMED@ 7:51pm 02-25-2013

College/Institute and Class

Surendranath Evening College
Sir our all college tests are over today. I want to ask you that when will college started for our electronics classes?
Replied on: 12:15pm 02-27-2013

1st Year Test examinations in other subjects are still on and would be over on 1st March..have a look at the college notice.We would resume formal classes on 2nd March, since as you all know, it is difficult to have the department kept open during such periods.

Goutam Das@ 10:41pm 01-29-2013
Sir please help me to debug this program to find the roots by Newton-Raphson method.. The program is given below..

#include <stdio.h>
#include <math.h>
int i;
float x0,x1;
float f(float x),fpr( float x);

printf ("Enter the value of root for trial solution: ");
scanf ("%lf",&x0);

for(i=1; i<=1000; i++)
x1= x0-f(x0)/fpr(x0);
if (fabs(x0-x1)<0.001)
printf("x0=%f x1=%f",x0,x1);


printf("The root is: %f",x1);


float f(float z)
float g;
g= z*z - 25.0;

float fpr(float z)
float gpr;
gpr = 2*z;
Replied on: 2:06pm 01-30-2013

x0 is declared to be of type use %f instead of %lf in the scanf statement

You have used ANSI prototype declaration of functions f and fpr..if they are not supported by your compiler, try out the old methods..

float f(), fpr(); // function declaration

float f(z)
float z;

float fpr(z)
float z;

Show me the programs in the class.

Joy Goswami@ 7:31pm 11-24-2012

College/Institute and Class

BE 7th sem.,Jorhat Engineering College,Jorhat,Assam
I am really impressed by your teaching methodology... Thanks a lot!
Goutam Das@ 11:12pm 10-31-2012

College/Institute and Class

Sir, please give any information about the circular for the INC results. All Students of asutosh college have already filled up their circular form on 31st october. Please help.
Replied on: 11:00am 12-02-2012

I had posted a note on my home page

SWARUP MUKHERJEE@ 12:28am 10-05-2012

College/Institute and Class

SIR,i'm in 1st yr of in ee,at rajabazar sc.colg .In future if i wish to become a professor at a engg colg. then how should i proceed and what is the way of amisssion to at sc. college ?
Replied on: 10:57am 12-02-2012

Well, you must appear in the GATE examination and strive to obtain a good score. Being qualified in the NET (or SET) examination may also help you stand out from the crowd. Some institues may require you to have an M.Tech degree or even a PhD degree. In any case, faculty are encouraged to obtain these degrees even after joining if they do not have them at the outset. Your performance in the semester examinations will also be considered, so do not neglect your studies. And finally a good performance at the interview will always be needed. Chances are that you would be tested more on the fundamentals than on the intricate aspects of the topics studied in your B.Sc/M.Sc./BTech/MTech.

As for your second question, I am sure your professors and seniors would be able to help you.

ABDULLAH ZIA@ 3:34pm 09-27-2012

College/Institute and Class

Why is the electrical conductivity of a pure semi conductor increases with increase in temperature?
Replied on: 11:12am 12-02-2012

In a pure or intrinsic semiconductor, the free electrons and holes are created in pairs when valency electrons gain thermal energy from the surroundings (T being > 0 degrees Kelvin), break covalent bonds and overcome the band-gap to reach the conduction band where they become free to partake in conduction. In the process they leave behind holes in the valency band. As the temperature is increased, valency electrons lying deeper in the valency band, also gain sufficient energy to cross the band gap, as a result of which the concentration of free electrons and holes increases.
The electrical conductivity is proportional to the concentration of free electrons (n_i) and holes (p_i). Note that n_i=p_i since the free-electrons and holes are created in pairs. So as temperature rises, n_i or p_i rise, which in turn increases the conductivity and also the current.

In this context it would do you good to recollect the relevant expressions for electrical conductivity (sigma), current density (J) and the intrinsic carrier concentration (n_i) (refer Streetman's or Millman Halkius' book)

sigma=n|e|mu_n + p|e|mu_p (for intrinsic semiconductor n=n_i=p=p_i)



Note that as T increases, not only does the exponential term decay more slowly, but also T^3 term increases faster, the net result being an increase in n_i.

KaustaviRayChaudhuri@ 10:37am 07-28-2012

College/Institute and Class

sir,i want private tutors or coaching centres' names regarding study of electronics honours,maths (general) & physics (general).
my name is Kaustavi Ray Chaudhuri
Replied on: 10:51am 07-31-2012

Sorry to dissapoint you in this regard!
You are however free, like everybody, to post your questions on my open guestbook on any academic topic. I will try to help if I can. Also, do visit my portal:, where you will find notes on many topics. You might suggest the names of other topics too which may be included.

Goutam Das@ 11:57pm 07-24-2012
I went to the college but there was not such notice about starting of the class of 3rd year. May we start classes from the next week?
Replied on: 10:40am 07-31-2012

I haven't come across any notice either! We may start to take classes need to come to college to know the schedule.

Mainak Bandyopadhyay@ 10:12pm 07-21-2012

College/Institute and Class

Dinabandhu Andrews College
Sir, this year I have admitted in Electronic Hons. at D.A. College. I will be highly oblige if you give me the name of some reference books and as well as advice me how I will prepare for the same.
Replied on: 4:45pm 07-22-2012

Books tend to be remembered by their authors, so I am giving you the names of the authors below:

For basic electronics: Chattopadhyay Rakhhit; Boylstead Nashelsky; Malvino Bates; Millman Halkius etc.
For Digital Electronics; Morris Mano; Malvino Brown; Malvino Leach etc.
For semiconductor physics; Streetman; Sze; Neaman etc.
For Microprocessor (8085): Gaonkar etc.
For electrical circuits: Yearwood; Sudhakar Shyammohan etc.
For instrumentation: Kalsi; Cooper etc.

The above list is not exhaustive..Ask your class teachers, they could name some other good books as well. Do not rush to buy all the books as most of them should be available in your library.

BTW, you have chosen a good subject with plenty of prospect and a good college too...Do convey my regards to your teachers.

Best wishes...
Dr. Abhijit Poddar

Soubheek Nath@ 1:46pm 07-15-2012
Sir, I just applied for a admission test in Jadavpur University on M.Sc in Instrumentation.. Bt I cann't find the syllabus for this test on their website or anywhere.. Please refer me which do I follow for this test.. The admission test will be held on 4th august..
Replied on: 4:30pm 07-22-2012

Since it is an admission test for admission to an M.Sc. Course in Instrumentation, I guess you would be asked questions mostly from B.Tech. in Electronics and or Electrical Engineering. Since you have been a student of B.Sc. Electronics, I suggest you revise well the topics pertaining to Electrical circuits, Electronic circuits and devices (especially OP-AMP theory and applications) and whatever electronic instrumentation you have learnt.
Usually you are tested on the basics, so your understanding of the fundamentals should be strong.
Hope this helps..
Best wishes.

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